When Will the Saltwater Wedge Reach New Orleans?

Saltwater is pushing its way up the Mississippi River and is expected to contaminate most of the New Orleans public drinking water supply on Oct. 28. President Biden declared a state of emergency on Wednesday, releasing federal funds to Louisiana. Officials are concerned about saltwater corroding lead pipes, which could leach heavy metals into the water system.

The Mississippi’s current is typically strong enough to prevent seawater from the Gulf of Mexico from moving upriver. But it’s a challenge that gets harder each year as global warming causes sea level in the Gulf to rise while coastal subsidence causes the riverbed to sink. The equilibrium broke down as the Midwest endured another exceptionally hot and dry year. Freshwater river levels fell, allowing the seawater to march upriver. The broader Mississippi watershed has endured consecutive years of drought. According to data from the U.S. Drought Monitor, dry conditions have persisted across western regions of the upper basin since 2020. Since May, those drought conditions have deeply affected the states in the Midwest, which feed the river downstream. “It’s really the areas in the Midwest, in the Ohio Valley, that really play a role in whether we get high or low water conditions on the lower part of the Mississippi,” said Jeffrey Graschel, hydrologist at the National Weather Service in Slidell, La. The lower Mississippi is isolated by levees and doesn’t have large tributaries feeding into it. “When it rains in the state of Mississippi or in Louisiana, it doesn’t actually go into the river at all,” he said. Below average rainfall and a record-hot summer have parched the central and eastern parts of the Mississippi River Basin. The western areas have had rainfall, but much of it is captured in reservoirs and doesn’t reach the headwaters of the Mississippi River in Minnesota. Precipitation over the past 6 months compared with 1991-2020 Source: PRISM Climate Group at Oregon State University In a year of record-breaking heat worldwide, much of the basin has also been much hotter than average. According to NOAA’s National Centers for Environmental Information, Louisiana just had both its driest and hottest August on record. The next few weeks in the lower Mississippi Valley are expected to be a continuation of that hot and dry weather, although the prospects for rain are higher in seasonal outlooks through the end of the calendar year. While there’s a good chance of some rain in the next two weeks, according to the National Weather Service precipitation outlook, a huge amount will be needed to stop the saltwater creeping upriver. Because salty water is heavier and sinks while freshwater flows over top, it creates a wedge shape as the mixture moves upriver. The Army Corps of Engineers is currently raising an underwater sediment shelf near Myrtle Grove as part of the mitigation response, since even a submerged barrier could block saltwater at the bottom of the river. But ocean water has already crept over the top of that sill and through a notch cut out for ship traffic. “The sill augmentation is not going to stop the saltwater from progressing upriver,” said Matthew Roe, spokesman for the Army Corps. “It’s just going to delay the impacts that we see, and give the parishes and state more time to get their plans in place.” Residents downstream have already had to switch to bottled water while the Army Corps tries to secure a plan to ship fresher water from upstream to lower the salinity levels to a point where water can be treated for public consumption. The low-water season typically runs from May through the end of October. Once temperatures cool, the river should start to rebound. “In a typical year, we typically don’t see low water conditions improving for our area until we start to get into November,” Mr. Graschel said. By then it may be too late, because the saltwater will already have hit the intake valves for the New Orleans public water supply.